The famously hot parts of the London Underground could become a lot more pleasant to use if a new way of cooling the tube proves to work, and the money is found to install it in tube stations.

When the tube tunnels were first dug they were so cold that the fact that the tube offered a respite from the hot sun was a selling feature in early adverts. However, over the past century, the cool clay that surrounds the tube tunnels has absorbed so much heat that it can’t soak up any more heat, and with nowhere for the heat to go, the tube trains have become uncomfortably hot to travel in.

Transport for London has been looking for a way of removing the heat for decades, and with such a large and varied network there was never going to be a one-size-fits-all solution, and now TfL has a new idea to try out.

Long term readers of this website will be familiar with the cooling the tube project, and also how cold groundwater from under Green Park is used to cool that tube station.

The Green Park trial worked in that it cools, but it turns out that the large fan units installed above the platforms are less than ideal for use in a live tube station. The main problem is one that anyone who has been on a Hidden London tour will recognise – and that is dirt. The tube tunnels are filled with dust and particulates being blown around, and those clog up the fans and ducts used in the cooling units. That means a lot more maintenance than is ideal, and being in a live train tunnel, it’s a lot more work to safely climb up to the fan units to clean them when the station is closed.

What was needed is a way of delivering lots of cooling with minimal fanning.

About 5 years ago, TfL senior engineer, Tony Ridley was asked to think about how to solve this problem, and after some thinking and computer design work came up with an idea that was interesting enough to secure some internal innovation funding.

What’s been created is a curved metal panel unit that can hang from the ceiling of a tube station with a series of aluminium rectangular tubes running through them. The large spaces in the rectangular ducts are for air to be pumped through, and the air is cooled by a supply of cold water that runs in pipes built into the structure. A key innovation is that running the cold water in parallel to the airflow reduces dirt build-up inside the units.

Although the cooling panel can be fitted with a fan directly to it, as in the test unit, the ideal model would be to put the fans in a cupboard somewhere else and pipe the air into the panels through ducting pipes. The key advantage is that the cooling panel is itself pretty much an inert component, made up of just the metal pipes, so there’s hardly any mechanical equipment to maintain at height. That should considerably reduce maintenance work at height, and the maintenance staff can carry out work on the fans stored in a cupboard somewhere more sensible.

That’s the theory – but does it work?

Although a small-scale model was built and the theory modelled on computers, it wasn’t until they built a full-size prototype at their office in Acton that they could see if theory matched practice.

And it exceeded expectations.

That was enough to move on to the next stage, which is a live installation inside a tube station for testing. And the disused platform at Holborn station is where TfL does tests like this.

The cooling panel demonstrator project has been 70% funded by the Department for Transport and Innovate UK as part of the Government’s TIES Living Lab programme. Chesterfield based Direct Engineering / INAL manufactured the cooling panel, and Essex based OCL installed it at Holborn station.

Walking through normally locked doors onto the long since disused Aldwych branch of the Piccadilly line, the empty platform is a bit cooler as it’s not in use, but it’s still warm, and at the far end is the test unit.

And the moment you walk under it, it’s as if walking into a bubble of cold air. It’s genuinely remarkable how the metal unit above our heads that has hardly any moving parts has created a patch of cold air that reaches right down to the platform.

This unit has a fan attached for testing had the fan turned up to full power while I was there and it’s no surprise that people tend to linger a bit in the cold air blasting out of the unit. Although not intended to blast cold air out of the unit when in public areas, as this is just a test unit, there could be an option to do that if it was felt necessary. One of the test engineers confessed that at times he has to move away while checking temperature readings in order to warm up a bit as the cooling is that effective.

Tony said that it was exciting to see his computer designs appear as a physically built machine in a tube station, admitting though that he was more relieved than excited when it was first switched on and proved to work as expected.

The heat being removed has to go somewhere, and as this is a test set-up, the water cooling unit is on the same platform in an empty cupboard for monitoring and blows the heat back into the platform.

That’s indirectly useful, as a rule of thumb to whether something is cooling a room is not to stand in front of the fan, as they always feel nice, but feel the heat being removed at the other end. If the heat exchanger is actively removing heat, that’s what cools a room, or a tube station, and here the heat coming out of the heat exchanger is very noticeable.

So the system works.

But still only in theory

They still need a fan to suck air into the unit, and that air will still contain dust so they want to be sure that the ceiling unit won’t clog up with dirt. The air channels are large enough that shouldn’t happen, but they need to be certain. Also if they do clog up, they need check in practice how easy is it to clean with the TfL equivalent of a long feather duster.

Fans are still needed to blow air into the cooling panels, but to help reduce dirt clogging up the fans, they’ve turned to the jet engine for inspiration. To reduce dirt intake, the fans only suck in about a third of the air needed, and the airflow behind the fan is used to suck in the remaining two-thirds of the air that’s piped through the cooling panel. The current plans are to test the fans in a live tube tunnel for a number of months to make sure that they cope with the dirty environment.

They also want to refine the design of the unit and work out what final consumer finishes will make it look more appealing in tube stations. It took a week to install this first unit, but the lessons from that will be included in future models, and they expect to get the installation of the cooling panel down to a single weekend.

In the long term, unless something totally unexpected happens to show they don’t work, then these panels could be rolled out across the tube network bringing much needed cooling to insufferably hot tube platforms.

One of the other niceties of the design is that it’s modular — so that several of them can be strung along a platform rather than being in a single fixed location, and they have a lot of flexibility in how they pipe air and cold water into the unit.

They also need to remove the heat from the station, and while off-the-shelf water cooling units do the job and can be mounted on roofs to get rid of the heat, they would ideally prefer more environmentally friendly ways of doing that. That could see, for example, existing cooling systems repurposed to use these cooling panels instead, and also the heat could be of use to people, such as the Bunhill energy network that uses Northern line tunnel heat to generate electricity.

There are also opportunities exist to add new cooling equipment when stations are upgraded to support the cooling panels. That’s exactly what’s happening at Knightsbridge station, which is being upgraded at the moment, and TfL has included a lot of cooling piping into the new oversite building so that they can more easily add a cooling unit into platforms.

Knightsbridge hasn’t been chosen as a possible first live deployment just because it’s being upgraded, but specifically because it’s on the Piccadilly line. A lot of new trains will be arriving in a few years time, and while the trains use about 20% less power than the existing trains, and put out a lot less heat, there’s also going to be a lot more trains running on the network. More trains inevitably means more heat, and how to deal with that is a key component of the Piccadilly line upgrade project.

Although currently unfunded, TfL aims to eventually increase train frequencies on the Piccadilly line to 33 and then 36 trains per hour. It is at this point that additional cooling at five Piccadilly line stations would be necessary, according to TfL’s modelling. The five stations would be Knightsbridge, Green Park, Holborn, Leicester Square and Piccadilly Circus. TfL could then identify other locations where the panels could provide a benefit.

What’s really nice about this trial is that it’s based on an idea from someone inside TfL given the space to think about the problem and a lot of support not just from senior management, but also from the engineers on the ground who saw an opportunity to try out something innovative.

Unfortunately, at the moment there’s no money to pay for the cooling system to be expanded beyond the current trial.

However, if the trials conclude as hoped and the money is found, then a journey on the tube could one day be as cool as it was for our Edwardian ancestors a century ago. Bring a jumper.


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  1. Sean says:

    Looking forward to the “TfL equivalent of a long feather duster” being sold at the LT Museum Shop when this eventually all takes off!

  2. Greg says:

    It’s interesting how the cool tube network became hot today.
    How about the Northern City (or Moorgate) Line?
    It was built quite long ago, and it’s still very cold.
    Is it because of the larger tunnel profile? Or different materials?

    • graham says:

      difference with the Northern City Line is it doesnt see the volume of trains per hour like the Deep Level Tube lines do, infact, if you average the frequence to 30TPH on alot of DLs during Peak time, thats alot of trains, another factor is the Deep Level Tube trains add heat to the tunnels and stations due to heating being used in the outer sections in winter, if they kept the heating off Deep Level trains, and let them run cold (as we wear warm clothing anyway) this would also help reduce the amount of heat being dispelled, as the trains after the first run through the tunnel would give them a moderate level of comfort in the outside, but its likely they would actively reduce the heat in the tunnels as they would take the heat away in the end. Another issue is the amount of metal cladding on deep level tube stations which trap the heat within the platforms, its noticeable how the JLE which uses concrete tunnels does not have such warm sections like the iron lined sections on the historic tube lines from the late 1800s to early 1900s. another issue is the fact Deep level tubes no longer have sufficient amount of opening windows on the carriage which would also help ventilation. A window at just the connecting ends of carriages means less ventilation, also remove the heating units from all Deep Level and fit an air cooler unit into the trains too

    • ianVisits says:

      The tunnel lining is nothing to do with the temperature – it’s the level of heat build up in the clay surrounding the tunnels. Unsurprisingly, newer tunnels have had less time to dump heat into the clay than the older tunnels have had.

  3. Jonathan Wadman says:

    How is the water kept cold?

  4. Jamie Burn says:

    Amazing! Cooling the tube is a wicked problem that we never quite cracked while I was running the programme – great to see some new ideas being trialled!

  5. Andrew says:

    The air in the Tube is filthy. It is bad enough breathing in that soup of particulates – another reason why facemasks may be a good idea – but I’d not want to have it blasted at me.

    Have they considered adding some sort of filter to remove the particles – perhaps a vortex, or electrostatic?

  6. SteveP says:

    Interesting. Of course, there is no such thing as “cold” – just an absence of heat. Heat removed from cars or platforms has to go somewhere. Anyone who has travelled on NYC’s air-conditioned cars in the summer will have been struck with the blast of superheated air that is deposited on platforms – like opening the oven door.

    One positive thing is that cold water can cool a lot of air – but where does the cold water come from? I’m sure TfL can figure out how to deliver chilled water from the surface to deeper parts of the Tube. Ironically, the issues here are similar to the “portable AC unit” limitations – again – heat has to “go” somewhere.

    I suspect the issue here are a bit hard to unpick – what is the objective? Cool cars? Cool platforms? Energy efficiency? Probably all of the above

  7. Aleks says:

    As Andrews comment about dirt. UK Transport is legendary for the lack of systemic co-ordination. Air pollution is a recognised London problem but accepted below.
    This is an engineering solution to a micro-problem of dirt affecting fan performance. The answer must be to clean the air before the fan with a filter. This is being avoided or ignored as either out of scope or cost. Implementing a cooling solution is dependant on an acceptable budget.
    Filters might need to be changed and washed daily DUE TO the high dirt levels. There probably already exist self washing and cleaning filter systems especially as a water supply is proposed.
    Costs are already being borne by fan cleaning and tunnel fluffers, platform staff, clothing cleaners and the health service. Effect on overall levels is minimal however it is magnified by the forced ducting. Active fans are an opportunity to begin, the additional energy load on the fans will use marginally more power and wear using solar offset.
    It is not a solution to remove all dirt but an indication of the direction of travel to a cleaner future.

  8. Jeff says:

    I’m a big fan.

  9. Andrew says:

    If the air is so dirty that it clogs the air cooling system then what is it doing to people’s lungs? They should look at this as an opportunity to help filter the air on the platform, the issue is just about putting the air intake and filter at an accessible location for frequent maintenance. As for cost benefit analysis, they could look at the value of increased health therefore reduced costs to the NHS. However, I suspect we would be lucky to get that type of joined up thinking.

    Btw, fascinating article and I am glad to see it looks like they are making progress on this issue.

  10. Lozebeth says:

    Would be wonderful not to arrive at my station wilted and panting. I’d much rather arrive shivering (one can always put clothes ON…). Let’s hope the
    Government wakes up and funds TFL as they need and deserve.

  11. Lionel says:

    Don’t know if this has been mentioned and that’s about catching two birds with one stone: how about taking out the filthy dust while you’re at it and help create a healthier environment for the waiting passengers.

  12. James S says:

    I’m not quite sure that this solution is tackling the right problem. Personally, I rarely experience great heat on the station platforms, usually just on the trains themselves. Maybe I’m just lucky with the stations I use most.

  13. Magdi Rashad says:

    This is known as chilled beams. It is manufactured by many and I have recommended using them in Pfizer pharmaceutical HQ in sandwich Kent in the 1990s ( They can even be combined with lights and fire sprinklers. A manufacturer from Finland produce them by the tons. It is a form of radiative cooling. One draw back is condensation in humid conditions, but I don’t think the conditions in the underground would encourage condensation for being low dew point high sensible heat environment

    • ianVisits says:

      No, these are not chilled beams or radiant cooling – they are a different product entirely.

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